1. Field of the Invention
The present invention relates generally to a radio receiver, and is directed more particularly to a radio receiver of a heterodyne type.
2. Description of the Prior Art
Recently, the number of communication stations which utilize transmitting and receiving apparatus and the like has increased. Even so, it has sometimes been difficult to find an opposite station. To avoid this difficulty, some radio receivers have recently been equipped with an automatic station selecting device which performs a so-called automatic scanning function which sweeps channels or stations to automatically select a desired channel. The automatic sweeping and stopping is controlled by detecting the existence or non-existence of a sensed signal in the desired channel. In the conventional receiver, the discrimination is carried out by the control signal which is obtained by, for example, an IF signal. The above discrimination, however, is only possible in the existence of the IF signal. Thus, the conventional receiver may receive such a signal as a weak electric field or the signal may contain much noise and is difficult to be heard. For example, if a noise whose frequency is the same as that of an IF signal is accidentally present in a receiver during a short period of time, the receiver is tuned to the frequency even though there is no signal to be received.